On Wednesday 29 June 2011 05:51:07 AJ MacLeod wrote:
> On Wed, 29 Jun 2011 10:39:55 +0100
>
> Vivian Meazza wrote:
> > > * English Electric Lightning: I believe the plane should be able to
> > > reach Mach 2 around 36.000 ft in level flight - even in a descending
> > > pattern, I was never able to reach above Mach 1.7 - is there a problem
> > > with the FDM?
> >
> > The author is inactive at the moment. Perhaps he can be persuaded to give
> > this fine model a bit of a work-over.
>
> The author is a bit too active, which sadly doesn't leave any time for
> Flightgear :-( The instruments I'll try and fix myself, but do any JSBSim
> experts have any ideas on what might have changed the behaviour of the FDM
> model? It worked OK "before"... though I have no idea when that changed.
>
> Cheers,
>
> AJ
I don't know of any changes that would have impacted your model. We did fix
the thrust tables a few months back... Its really hard for me to see
individual aircraft histories in git, but we probably fixed the elevator drag
issue in this FDM as well. If you remember, aeromatic configured CDde to go
negative at times causing false thrust.
I looked at this model in JSBSim stand-alone and saw three sources of drag at
mach 2.06 and 36,000 ft:
aero/coefficient/CDalpha = 12993.134843 (pounds force)
aero/coefficient/CDmach = 5197.253937 (pounds force)
aero/coefficient/CDde = 8567.342560 (pounds force)
A combined 26,700 pounds of drag.
Our two engines in full reheat at mach 2.06, 36,000 feet put out a combined
thrust of 20,200 pounds of thrust. Leaving 6,500 pounds of drag unbalance, so
we slow down.
The CDde coefficient is 0.04 * |elevator-pos-norm|. This is an aeromatic
default value for all aircraft. Since the Lightening F.1a has an all moving
tailplane not an elevator, this number is way too big. Changing it to 0.008
yields an output of:
aero/coefficient/CDde = 1787.161206 (pounds force)
Or a combined 20,000 pounds of drag. Just about matching what our engines
produce.
Thanks,
Ron
------------------------------------------------------------------------------
All of the data generated in your IT infrastructure is seriously valuable.
Why? It contains a definitive record of application performance, security
threats, fraudulent activity, and more. Splunk takes this data and makes
sense of it. IT sense. And common sense.
http://p.sf.net/sfu/splunk-d2d-c2
_______________________________________________
Flightgear-devel mailing list
Flightgear-devel@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/flightgear-devel
